1. Field of the Invention
The present invention relates to a belt unit in which a belt is supported by a plurality of rollers including a driving roller and a driven roller, a roller which can configure the belt unit, and an image forming apparatus which forms an image on a record medium while conveying the record medium by the belt unit.
2. Description of the Related Art
Heretofore, in a belt unit in which a belt is extended between a driving roller and a driven roller, various configurations have been proposed as measures for preventing a situation where the belt shifts in the widthwise direction thereof, that is, where the position of the belt shifts toward either end side of each roller (for preventing the occurrence of what is called the “deviation of the belt”).
There has been, for example, a configuration which includes a guide portion that protrudes from each of both the end parts of the belt in the widthwise direction thereof, toward the inner peripheral side of the belt, and a shift prevention portion that is arranged in a positional relationship where it comes into touch with the corresponding guide portion having shifted together with the belt toward the end side of the roller, and in which the shift of the belt is restrained by the shift prevention portion in cooperation with the guide portion.
There has also been a configuration in which, for the purpose of more stabilizing the state of the belt, tensions that the belt undergoes in a state where this belt is extended over the rollers are made different on both the end sides of each roller, whereby the belt is allowed to shift to only one end side of the roller, and a touch member is disposed in a positional relationship where it can restrain only such a shift.
Considered as a practicable example of the latter is a configuration in which spring members are respectively mounted on both the ends of one roller, the spring members pull the belt by forces different from each other, in the direction of coming away from the other roller, and the magnitudes of the tensions to be exerted on the belt by one roller are made different at both the ends of this roller, whereby the forces of shifting the belt are always exerted in directions in which the shifts of the belt are prevented by the touch members.
However, the spring members for use in such a configuration are structurally difficult to be manufactured so as to have precise pulling forces, and consequently, large errors of or above ten odd % develop in the pulling forces. Therefore, the pulling forces need to be endowed with a large difference in order that the pulling forces at both the ends of one roller may not be reversed under the influence of the errors. Thus, large forces are exerted on, not only the rollers, but also the belt, to give rise to a state where a large load acts on the whole belt unit. Therefore, the configuration cannot be the desirable from the point of the durability of the unit.
In recent years, accordingly, various configurations which do not employ such spring members have also been proposed.
By way of example, there has been proposed a technique (refer to JP-A-08-190319) wherein, on one end side of one of a plurality of rollers (support rollers) constituting a belt (a paper pressing belt 402) (on the side of one end 403b of the roller in the axial direction thereof), a shift prevention portion (a flange portion 403c) which restrains the roller from shifting (deviating) toward the belt is disposed, while a tapered portion (403d) which has an outer diameter larger than on one end side (that is, whose section has a length d2 greater than the length d1 of the section of one end 403b) is disposed on the other end side of the roller. Thus, the magnitude of a tension to act on the belt (to urge the belt) is made smaller on the other end side than on one end side so as to exert the force of shifting the belt, in a direction in which the shift of the belt is prevented by the shift prevention portion.
Also, there has been proposed a technique (JP-A-2002-268455) wherein a belt (an endless belt-shaped photosensitive member) is extended between two rollers which are inclinedly arranged so that the peripheral length of the belt may become greater on either of both the end sides of the belt in the widthwise direction thereof, and a guide portion (a deviation stopper guide) is disposed at that end part of the belt at which the peripheral length is greater. Thus, the magnitude of a tension to act on the belt is made larger at one end on the side of the greater peripheral length, than at the other end, so as to always exert the force of shifting the belt, in a direction in which the guide portion comes into touch with the end part of each roller and prevents the shift.